Most of the conventional bicycles are configured to be foldable for easy transportation. However, many of these conventional foldable bicycles require complicated folding system to transform the bicycle from unfolded state to folded state. For example, in U.S. Pat. No. 6,196,566, the foldable bicycle is needed to form three quadrilaterals first in order to be folded up. The folded state of the bicycle in that patent is relatively bulky because only the four bars forming the two triangles of the bicycle frame can be folded up but not the front/rear forks or the handle bars. Some other conventional foldable bicycles may have a folding system at the middle of the crossbar in order to reduce the length of the bicycle into half when it is folded up, for example, in U.S. Pat. No. 3,865,403. However, that kind of folding system at the middle of the crossbar is unable to withstand strong force against the bicycle frame when the bicycle is in use. A quick, easy to fold, non-bulky and lightweight frame which can withstand strong force in a bicycle is therefore desired.